US2803576A - Method of forming fiber reinforced fabrics - Google Patents

Description

Aug? 1957 'c. DONALDSON 2,803,576

' METHOD OF FORMING FIBER REINFORCED FABRICS Filed April 11,, 1952 FIG. 1..

' 40% RESIN LOADED l-PLY OF PARALLEL FIBERGLASS ENDS FIG. 3,

Mum- P-LY TUBE IN VENTOR CHASE DONALDSON ATTORNEYfi United States Patent METHOD OF FORMING FIBER REINFORCED FABRICS Chase Donaldson, Washington, D. C.

Application April 11, 1952, Serial No. 281,859

3 Claims. (Cl. 154-93) This invention relates generally to laminated fabrics and the method of making the same and more particularly to fiberglass reinforced resin laminated fabrics, pipes and tubing, and other items and to the methods of forming the same.

Various methods of forming fiberglassreinforced resin fabrics and tubing are known in the art but none of these methods or products have been successful from a commercial standpoint for a wide variety of reasons.

For example, the material most widely employed in prior art practices is woven fiberglass cloth which has an objectionably high cost per pound, must be heat cleaned to remove the initial starch from the cloth, and requires the application of finishes to enable the resin to adhere to the cloth to a greater extent.

Another prior art method involves the use of a fiberglass mat formed of random fibers which is less expensive than woven cloth. However, irregular and hence unsatisfactory results are obtained, especially in the formation of tubing, due to the difficulty of controlling the quality of the random fibers mat as to their distribution and weight.

A further and more recent prior art method known as filament winding has produced results which are more satisfactory from a strength standpoint but which are still objectionable in that irregularities are produced with a high proportion of rejects and porosity in the tubing.

This operation consists in the impregnating of fiberglass yarn or roving with resin at the time of manufacture of the tube and winding it into a revolving mandrel using a multiplicity of heads, each of them carrying one or more fibers or strands of yarn. The disadvantages of this method are three-fold. It is not a continuous process in 'that only a given length of tubing can be made depending upon the length of the mandrel employed. Fiberglass yarn does not accept resin impregnation readily, or rapidly and the speed of the process is somewhat limited to the rate at which the yarn can be successfully impregnated, whether through a bath process, speed rollers, or otherwise.

Accordingly, the chief object of the present invention is to provide an improved reinforced laminated fabric, tubing, etc. and method of forming the same which will obviate the above mentioned disadvantages characterizing the prior art products and methods of formingthe same.

Another important object of the present invention is to provide an improved reinforced laminated fabric, tubing, etc. and method of forming the same which will be of higher bursting strength than that formed by known prior art methods and of lower cost, susceptible of ready control in manufacture, and durable in use.

Other objects and advantages of the present invention will become apparent during the course of the following description.

In its broadest aspects, the invention contemplates a laminated fabric, tubing, etc. and method of forming the sarne fromfia highly' absorbent paperwhich willreadily 2,803,576 PatentedAug. 20, 1957 accept a binder at high speeds and then restrain in parallel patterns, the reinforcing yarn deposited thereon.

In the drawings, I have shown two embodiments of the invention. In these showings:

Figure 1 is a perspective view of a laminated fabric formed in accordance with the method comprising a part of the invention;

Figure 2 is a perspective view of a similar fabric formed of plural laminae; and

Figure 3 is a perspective view of tubing formed in accordance with a modification of the invention.

Figure 4 is an end view of tubing formed by convolute rolling of the laminated fabric.

In the practice of my invention, a dense highly absorbent paper, thoroughly preimpregnated with uncured resin, has continuously laid thereon any number of ends of yarn per inch up to a maximum possible depending upon their diameters. The thinness of the paper is highly important in order to add the least amount of bulk to the end product and to obtain a higher proportion of reinforcing yarn fibers so as to increase its strength. The paper is preferably also highly absorbent so that it will pick up and accept the adhesive or binder, such as resin, from conventional coating equipment or impregnating rollers at high speed. Further, the paper functions to restrain the fibers in parallel patterns when they are bonded to the paper due to its loading with resin.

It is preferable to use a paper stock which is inert and in which there is no element or impurity which might inhibit or affect the cure of the resin. The strength of the paper is not important as the strength of the laminae comes almost entirely from the glass, etc. reinforcing fibers. For example, forty-five fiber ends per inch are laid on .003 inch thick No. 811. rag stock paper which tests about 200 lbs. pull and the paper alone contributes no more than twenty pounds to the overall strength. Thus, a reasonably good grade of asbestos paper, rag stock paper, or newsprint may be employed but kraft paper is preferred for its cheapness and relative degree of strength.

A paper which is in some instances preferred is one made of superfine glass fibers which have an insoluble resin-forming compound of the thermosetting type suspended in the pulp from which the paper is prepared.

The paper of very fine glass fibers, as contrasted with the ordinary fiberglass, has a disadvantage resulting from its low tensile strength, but after setting of the resin, a tube of improved physical properties, particularly from an electrical standpoint, may be obtained.

Resins employed For use as an adhesive or binder, a contact uncured resin of the thermosetting type which can be dissolved in acetone or other solvents is preferred. Such an uncured resin is preferably either crystalline in structure so as to be ground up into fine particles or in the form of a powder to insure good physical distribution. Two such resins with which particular success has been obtained is G-382, a polyfumarate made by the Atlas Powder Company used separately or in combination with diallyl phthalate made and distributed by the Shell Chemical Company. Two other resins which are acceptable for this same use are Epon series made by the Shell Chemical Company as well as certain resins made by the'Ciba Corporation under the name Araldite both of which are of the ethoxylene series. Certain polyesters may also be used advantageously. In general, the resins are preferably of the low pressure or contact type which may be set without the necessity of high pressure. The low temperature resins which set at substantially room temperature are not as desirable as those requiring higher temperatures for curing because of their lower final strength and resistance to distortion by heat.

It is essential with the above contact resins to preimpregnate the paper so that it has an adequate resin loading, up to 40% for fabrics. In the forming of tubing, additional resin is used up to an ultimate loading of 45% or 50% by weight if the tube is to be well laminated and non-porous. It is to be noted that this combination of a resin preimpregnated paper plus the use of an additional quantity of solutions of resins, commonly esignated as wet resins, compatible therewith is an important feature of the invention. The use of the preimpre'gnation resin which in itself is tacky to hold down the reinforcing fibers or yarn while it is being laid on, takes the place of the 58% adhesive coating previously employed. The degree of tackiness of the impregnated paper is a function of the compounding of the resins. For example, if a high proportion of resin known as diallyl phthalate has been used but is not over 25%, the end impregnated material is highly tacky, and What is known in the art as drapey. Furthermore, if a high proportion of .so-called flexible resins are used, the degree of flexibility of the preimpregnated paper and of the end product is increased.

Preimpregnation of paper stock The uncured resin is compounded with inhibitors, catalysts, etc. to effect the particular end result in question and dissolved in a suitable solvent which will depend on the particular resin. Often, and particularly with materials of high moisture content, such as asbestos paper, it is desirable that the paper be dried prior to impregnation to minimize the formation of voids on curing. The paperis then run through the resin bath, or

preferably, over a roller on which there is a spreader knife to secure uniform distribution of the resin on the paper. This suflices for preimpregnation due to the thinness and highly absorbent character of the paper combined with the good flow and penetrating characteristics of the resin solution employed. The flow depends upon the base viscosity of the resin, the fineness of the powdered resin employed, and the degree of dilution with solvents.

For normal operation, a -15% solution of acetone, i. e., 85% solid resin, has proven satisfactory and gives a sufiiciently tacky coated surface so that the fibers Will readily adhere thereto. This is accomplished at substantially higher speeds and with materially less irregularity in the lay-on of the yarn than with phenolic or other adhesives which are customarily employed. It is not necessary to sprinkle dry res-in powder on the coated surface following the lay-on of the fiber as a 40% resin loading remains in the paper after driving off the solvent. This resin loading may be varied from -40% on the initial impregnation. The resin, in turn can be loaded up to 20-30% by weight with inert material such as mica, silica, calcium carbonate or other fillers.

The balance of the impregnation is secured at the time of roll-up or lamination when a wet lay-up resin is employed and its viscosity is varied by the compound operation and by the use of a different viscosity to obtain the requisite flow of the wet lay-up or adhesive resin. 7 In some cases it is desirable to pass the paper through the .heated oven at a temperature above 212 F. prior to impregnation, .in order to drive off most of the moisture in the paper. Asbestos paper, for example, retains 2 /2% moisture which is-drivenoff at the time the solvent is ex- ,tracted. Better laminates may be obtained by driving off the paper moisture first by passing it through the oven and then impregnating in the manner described. The resin apparently takes the place of the moisture in the cellular structure of the paper and when set by thermal action, there is little tendency for the paper per se to absorb moisture as it would otherwise do.

Method of forming reinfonced, prcimpregnated fabric I ure 1, which may be stored or immediately used in the tubing, etc.

manufacture of laminated fabrics by uniting several sheets (Figure 2), tubing (Figure 3) or other products.

In the formation of laminated fabric, it is generally desirable to have the reinforcing fibers running in different directions, as illustrated in Figure 2, in the several sheets. The relative strength in the different dimensions of the paper can be controlled by the number of ends per inch in the separate sheets, or by the number of sheets having the reinforcing fibers running in one particular direction. While it is possible to apply reinforcing fibers running in different directions to the same preimpregnated sheet, it is generally preferable from an operational standpoint to have only fibers running in one direction on each preimpregnated sheet and control the strength in different directions during the lamination operation.

If the impregnated reinforced paper is to be stored for all or a portion of its estimated shelf life of 6-12 months, steps must be taken to prevent the rolls from sticking together. During the formation of the rolls, they are interleafed with polyethylene or cellophane which is rolled on simultaneously and which is correspondingly unrolled and removed in the manufacture of Another material which may be successfully utilized for interleafing purposes is an extremely thin absorbent piece of paper such as .003 inch unimpregnated rag stock which gradually absorbs resin from the adjoining separated sheets of the roll. A high strength tubing has been formed from .007 inch asbestos paper impregnated in this manner and reinforced with yarn. When stored, the paper from which the tube was formed was interleafed with .003 inch rag stock paper to keep the roll from sticking together.

In forming the impregnated reinforced paper above described, the paper webs can be up to any desired width during forming, depending on the width of the paper available, and then be split for spiral tube winding in the desired tape widths depending upon the tube diam. eters, or it can be wound up on a conventional convolute tube winding machine. Thus, the preimpregnated, reinforced and faced paper may be handled, insofar as the tube making equipment is concerned, exactly like kraft or other paper normally used in making paper tubes.

Method of forming reinforced fabric tubing A tube may be formed from the impregnated and reinforced fabric either by convolute rolling the fabric or by spiral winding a relatively narrow strip in conventional spiral winding tube forming equipment. In most instances, the tube will be formed on a metallic mandrel which supports the fabric during the winding operation. During the winding of the tube the tension on the fabric and the application of pressure from rolls aids in the formation of a dense, uniform structure. Generally, it is preferred to add additional resin while forming the tube to increase the amount of resin in the final structure.

The uncured resin may be sprayed on if of the wet type,

added as a dry finely ground powder directly between successive laminae in the tube rolling operation.

As stated, in the initial formation of the laminated paper in the tube rolling or laminating operation, the surface or the adhesive resin used between each ply is tacky. When rolled under light pressure or tension, and particularly if rolled with an additional wet resin, the air is squeezed out and replaced by resin. The surface tension or fluid friction, or in some cases, the tackiness and adhesion of the resin itself, causes the plies to stick together until such time as the tube is cured by heat which causes the resin to set. Thus, a continuous homogeneous tube is formed which does not delaminate under high impact of loading depending on the composition of the resin employed.

A pressure roll in the formation of convolutely wrapped tubing is desirable as it effects a more even distribution of the liquid resin being applied in the tube rolling operation which requires extreme care. The minimum roller pressure should be about 20-25 pounds per square inch depending upon the weight of the roll, its rotary speed, the diameter of the tube being rolled, etc.

Another feature of the method of spiral tube wrapping is to first form a standard paper tube with an external cellophane coating, to then carry on with successive laminations of the impregnated material, finishing off with another paper tube having cellophane, etc. on the inside, i. e., next to the preimpregnated material. These paper tubes serve to hold the item in place during curing and subsequently slip 01f. The use of cellophane, etc. either on the mandrel or outside of the tube that is rolled, be it spiral or convolute, is chiefly for the purpose of containing the resin within the tube. The ends of the cellophane wrapping may be taped to prevent such flow. Certain resins of the polyester type are, to a certain extent, inhibited by the presence of air during the curing operation; other resins such as the Epons are not so inhibited but do tend to fiow from the application of heat which eventually results in an air-tight cure.

In the formation of tubes to have high burst or compressive characteristics as in rockets, it is desirable to have twice as much strength in the hoop direction as in the axial direction. Twice as many strands of fibers are therefore laid convolutely or hoop-wise as axially which requires a cross laminating technique. This may be readily effected either prior to or at the time of the roll-up of the tube either by varying the number of layers to be crosslaminated, i. e., half as many cross-laminated as hoopwise, or by reducing to half the number of strands of fiber which may be laid in the axial direction, i. e., ninety ends to the inch in the hoop direction and forty-five ends to the inch for the paper employed for the cross-lamination in the axial direction as described in connection with the formation of the laminated sheet. The burst value may also be increased by employing a substantial proportion of the hoop-wise fibers next to the inner surface of the tube as stresses are highest there, decreasing toward the outer wall especially in thick walled objects.

The angle of the spiral winding affects the strength of the tube in either the hoop or axial direction. Theoretically, an angle of sixty degrees from the horizontal gives twice as much strength in the hoop as in the axial direction. The diameter of a tube will effect the width of the tape to be used in its winding. The greatest strength is obtained by an S and a 2 wind, i. e., one course righthanded and one left. Any number of courses or plies can be laid on in one direction and alternated with other plies in the opposite direction in accordance with the purpose and structural requirements of the tube.

Curing of the resin The curing of the tubes depends upon the particular 6 caused to set at the proper heat required for it, has stronger end characteristics than those cured at room temperature or below.

As an example, a resin compound comprising approximately G-382 and 20% diallyl phthalate requires about 280 F. for curing in five minutes with a 4; inch thick wall. This can be varied by varying the nature of catalysts compounded with the resin. Inhibitors which increase the shelf life of the resin and of the preimpregnated paper may also be used. The inhibitors act to inhibit the action of the catalysts until the appropriate degree of heat is employed and thus prevent the setting up of the resin under normal conditions of heat, pressure or long storage. Heating to the temperature necessary for curing can be accomplished by placing the tube in an oven, heating in a stream of air, heating the mandrel, or other means for obtaining the necessary temperature.

Another and preferred method of curing the tubes is to add a high concentration of promoter, either with or without additional resin, between the plies in the tube forming operation. The promoter activates the catalyst, probably by initiating an exothermic reaction which raises the temperature to a level at which the catalyst is effective. The particular promoter employed will, of course, depend on the catalyst and resin. Overheating of the resin can be prevented by control of the amount and type of the promoter.

The tubes formed are different in a chemical sense in that the filler, for example, actually consists of the asbestos paper employed in addition to such other fillers as may be used, the fiberglass yarn being a filler as well as a reinforcing medium. As a typical example, the overall composition of a finished tube may be approximately 45% resin, 25% glass, and 30% paper where one-third of the paper is rag stock and the other twothirds asbestos fiber paper.

Wherever the terms resin, fiber, paper are used in this specification and in the claims, they are intended to refer to any of the preferred materials set forth in this specification as suitable for the purposes of the present invention.

It is to be understood that the forms of the invention herewith shown and described are to be taken as preferred examples of the same and that various changes in the shape, size and arrangement of parts may be resorted to without departure from the spirit of invention or the scope of the subjoined claims.

I claim:

1. The method of making a composite sheeting comprising the steps of rolling a thin absorbent web into a roll with a contact resin coated web, permitting said absorbent web to absorb a coating from said resin coated web, unwinding said absorbent Web and depositing a layer of fibers thereon, and curing said absorbent web.

2. The method of making laminated fabric comprising the steps of rolling a thin absorbent web into a roll with a contact resin coated web, permitting said absorbent web to absorb a coating from said resin coated web, unwinding said absorbent web and depositing a layer of fibers thereon, assembling a plurality of the composite webs in superposed relation and uniting the assembled laminae by curing said absorbent webs.

3. The method of making laminated fabric comprising the steps of rolling a thin absorbent web into a roll with a contact resin coated web, permitting said ab sorbent web to absorb a coating from said resin coated web, unwinding said absorbent web and depositing a layer of fibers thereon, and forming said absorbent web into a roll with an untreated web in contact therewith to prevent adhesion of the roll as a unit.

(References on following page) References Cited 1'11 the file of this patent UNITED STATES PATENTS Meyer Nov. 21, 1899 Wheildon June 11, 1918 Moore Nov. 1, 1932 Cogovan et a1 Dec. 21, 1948 Stephens Apr. 19, 1949

Claims (1)

1. THE NETHOD OF MAKING COMPOSITE SHEETING COMPRISING THE STEPS OF ROLLING A THIN ABSOSRBENT WEB INTO A ROLL WITH A CONTACT RESIN COATED WEB, PERMITTING SAID ABSORBENT WEB TO ABSORB A COATING FROM SAID RESIN COATED WEB, UNWINDING SAID ABSORBENT WEB AND DEPOSISTING A LAYER OF FIBERS THEREON, AND CURING SAID ABSORBENT WEB.

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